Materials having high reflectance and reduced absorption in the near infrared region (NIR) of the electromagnetic spectrum (between 700 and 2500 nm) may be advantageous in many applications. For instance, products made from such materials tend to remain cooler under solar illumination and the lower temperatures can result in lower thermal degradation, improved durability, greater comfort, lower air conditioning costs, and reduced environmental impact.
High solar reflectance may be achieved in different ways. For instance, items with white outer surfaces may have high solar reflectance, however, this approach is unsatisfactory if a color is desired. For example, high solar reflectance and reduced absorption in the near infrared region may be achieved by combining conventional titanium dioxide pigments with non-NIR absorbing colored pigments and dyes.
When titanium dioxide containing products such as paints and plastic products are exposed to the sun, however, it is important that the product lifetime is not unduly curtailed due to deterioration following the sun exposure.
It is known that such outdoor/sun-exposed products containing titanium dioxide and other pigment fillers may not be photostable and can prematurely deteriorate via photochemical and photocatalytic reactions.
Although titanium dioxide itself does not degrade, the extent to which an item containing titanium dioxide degrades may depend upon the photocatalytic activity of the titanium dioxide pigment used in the item.
A coating layer of certain inorganic materials may be applied to titanium dioxide particles and pigment particles in order to reduce the photocatalytic activity. A coating with a silica layer may, for example, reduce the photocatalytic activity of titanium dioxide particles.
A dense or a fluffy SiO2 layer can be applied to titanium dioxide particles, e.g., such as described in: H. Weber, “Silicic acid as a constituent of titanium dioxide pigments”, Kronos Information 6.1 (1978). Coating with inorganic oxides, such as SiO2, ZrO2, SnO2, Al2O3, etc., can increase the photostability of TiO2 particles. An outer Al2O3 layer may improve dispersion of the particles in the end matrix.
One skilled in the art would expect a higher level of a dense silica coating layer to result in a greater reduction in the titanium dioxide pigment's photocatalytic activity. Levels of coating on the titania at amounts as high as 10 or 20 w/w % have been contemplated.
Current commercial products (e.g., those with a particle size of from 0.25 to 0.32 microns) may be coated with a minimum of at least 3 or 3.5 w/w % silica (or other inorganic oxide), together with at least 2 w/w % alumina.
High levels of the dense silica coatings have also been used to treat larger titanium dioxide particles. US 2014/0073729 A1, for example, describes doped titanium dioxide pigment particles having a mean particle size of from 0.4 to 1.0 microns. These particles are subjected to an inorganic surface treatment and/or organic surface treatment. The titanium dioxide particles are in particular coated with a 3 w/w % level of silica followed by coating with a 3 w/w % level of alumina.
EP 2 285 912 describes a coated particulate titanium dioxide material, wherein the material has an average crystal size of greater than 0.40 microns. The coating comprises one or more oxide material, for example, silica. In one example, 3% silica and 2% alumina are used to coat the particulate material.
Components of paints and products used in outdoor applications are becoming more photostable, but the cost of treating filler particles with coatings to make them suitable for such applications increases the cost of products.
There is a continuing need for inorganic particulate materials, such as titanium dioxide particles, with ultra-low photocatalytic activity to assist longer product lifetime of items exposed to the sun.